Multiple-input multiple-output (MIMO) wireless systems are those that have multiple antennas on the transmitter side and the receiver side. One way to exploit the number of antennas in a MIMO wireless system is referred to as “spatial multiplexing”, where different data streams are transmitted in parallel from different transmit antennas and where multiple receive antennas are used to separate the different data streams and the receiver. With spatial multiplexing, a node (eNB) may send multiple data streams (or layers) to user equipment (UEs) in downlink transmission using the same frequency. The number of such layers or streams for a downlink transmission is referred to as “the rank”. In Long Term Evolution (LTE) Release 8 standard, each UE estimates the downlink (DL) channel and reports the recommended rank indicator (RI) to eNB. Each UE also reports the channel quality indicator (CQI) and the precoding matrix indicator (PMI) which is an index to the precoding matrix in a codebook. These indicators provide a set of recommended transmission properties to eNB. Upon receiving this feedback (RI/PMI/CQI) from UEs, the eNB performs corresponding downlink MIMO transmission scheduling.
Implicit CSI (CQI/PMI/RI) feedback is based on a pre-defined set of codebooks, which are a set of matrices calculated offline and known at the eNB and UEs. The codebook of rank-r consists of a number of Nt×r matrices where Nt is the number of eNB transmit antennas. The UE feedback includes the RI, PMI and CQI mentioned above. RI refers to a preferred transmission rank (number of data streams), ranging from 1 to min(Nt,Nr), where Nr is the number of receive antennas. PMI refers to a UE recommended precoding matrix index in the rank-r codebook. For evolved UMTS Terrestrial Radio Access (E-UTRA) LTE Release 8, a single PMI is reported for each frequency subband, corresponding to the RI report. CQI refers to the quality of the channel (e.g., supportable data rate and/or signal-to-noise ratio). The reported CQI is associated with the reported PMI.